Mechanical presses such as a link press, a crank press, or an eccentric press are commonly used to produce automobile parts stamped or pressed from steel blanks. Todays large mechanical presses are most often driven by a flywheel. The function of the flywheel is to store the necessary energy to carry out a pressing operation. A relatively low power electric motor drives the flywheel so that before the start of a press operation the flywheel is rotating at the speed at which the pressing will occur. Mechanical servo presses drives of the “direct drive” or mechanical servo type use the power of a relatively high power electric servo motor to provide the energy for pressing. Another type of servo press is a “hybrid drive” type, or hybrid press. In this design the flywheel still provides the necessary energy for pressing, but power for the movement of the press is supplied in part by an additional servo motor, generally a smaller motor in a hybrid press than in a direct servo mechanical press.
In large press lines for the automotive industry, robots are often used to move parts from one press to the next. For optimal productivity, the motion of each unloading robot is synchronized to the motion of a press, and the motion of a loading robot is synchronized to the motion of the robot unloading the same press. The motion of the press itself can however not normally be controlled. Only the starting instant of the press operation can be chosen in relation to the motion of the loading robot.
The production cycle of a press as describe herein includes a pressing stage and a non-pressing stage. Stamping, pressing, punching, forming etc takes place during the pressing stage. After the pressing stage the non-pressing stage includes a first time after the press has begun to open in which a tool or robot may reach into the press and unload a workpiece that has been formed or pressed. After that and before the press closes a robot or other apparatus places a new workpiece in the press ready for the next pressing/forming operation. The objectives of the conventional type of synchronization of the press to the robot are commonly that:                the press should reach a certain point of its motion (called “Die Protect”, DP) during loading of a part or blank not before the robot has passed a certain point of its motion, principally the time when the robot loader has withdrawn from between the moving parts of the press. Failure to do so in time means a collision will occur, causing damage to the press die and/or the robot tool,        the press should pass the DP point as shortly as possible after the robot has passed the above certain point of its motion. Failure to do so means loss of time, i.e. reduced productivity.        
The synchronization mechanism of today is based on the assumption that the press is standing still in a well-defined position at the start of every press cycle, and that the press will then move with a pre-defined motion profile that cannot be varied. These operating conditions are a direct consequence of the mechanical construction of the press drive, which consists of a flywheel, a clutch and a brake. In contrast to that, servo press drives of either the “direct drive” or “hybrid drive” type, have fundamentally different constraints.
For example a servo press may be arranged with a drive that has a peak power of around 10 times the peak power of the motor that drives the flywheel in today's presses. Such a servo motor allows very rapid acceleration of the press from standstill to high speed. The press could thus be started and stopped in much the same way as a traditional flywheel-clutch-brake combination, and synchronization to robots would be very similar to the existing scheme. However, since such high peak power comes with unacceptably high drive and installation costs, one may instead use a press with a servo drive with much lower peak power. For example a peak power of the direct or auxiliary motor may be arranged to be between 1 and 4 times that of the traditional flywheel motor. A disadvantage of this approach is that accelerating the press from a standstill to maximum speed takes 1-1.5 seconds, which is much slower than accelerating using the clutch, and thus takes a significant part of the production cycle (which may typically be around 5 seconds total).